Nickel-titanium alloys are commonly used for the manufacture of intraluminal biomedical devices, such as self-expandable stents, stent grafts, embolic protection filters, and stone extraction baskets. Such devices may exploit the superelastic or shape memory behavior of equiatomic or near-equiatomic nickel-titanium alloys, which are commonly referred to as Nitinol. As a result of the poor radiopacity of nickel-titanium alloys, however, such devices may be difficult to visualize from outside the body using non-invasive imaging techniques, such as x-ray fluoroscopy. Visualization is particularly problematic when the intraluminal device is made of fine wires or thin-walled struts. Consequently, a clinician may not be able to accurately place and/or manipulate a Nitinol stent or basket within a body vessel.
Current approaches to improving the radiopacity of nickel-titanium medical devices include the use of radiopaque markers, coatings, or cores made of heavy metal elements. In addition, noble metals such as platinum (Pt), palladium (Pd) and gold (Au) have been employed as alloying additions to the improve the radiopacity of Nitinol, despite the high cost of these elements. In a more recent development, it has been shown (e.g., U.S. Patent Application Publication 2008/0053577, “Nickel-Titanium Alloy Including a Rare Earth Element”) that rare earth elements such as erbium can be alloyed with Nitinol to yield a ternary alloy with radiopacity that is comparable to if not better than that of a Ni—Ti—Pt alloy.
Ternary nickel-titanium alloys that include rare earth or other alloying elements are commonly formed by vacuum melting techniques. However, upon cooling the alloy from the melt, a brittle network of secondary phase(s) may form in the alloy matrix, potentially diminishing the workability and mechanical properties of the ternary alloy. If the brittle second phase network cannot be broken up by suitable homogenization heat treatments and/or thermomechanical working steps, then it may not be possible to find practical application for the ternary nickel-titanium alloy in medical devices or other applications.